Order of Magnitude: Use terms like “orders of magnitude” to describe significant differences of scale.

Negligible: When something is small, say it is “negligible” non-zero, but negligible.

Infinitesimal: If it is really really small, say it is infinitesimal.
Non-trivial: For a physicist, nothing is ever hard or difficult – it is always “non-trivial”

First-order approximation : That is only a first-order approximation to a good cup of coffee… “The living room is clean. Well…at least to a first order approximation.”

Canonical: Use “canonical” when you mean “usual” or “standard.” As in, “the canonical example of talking like a physicist is to use the word ‘canonical.’”

Orthogonal: Use “orthogonal” to refer to things that are mutually-exclusive or can’t coincide. “We keep playing phone tag — I think our schedules must be orthogonal”

Emprical Data: Any actual personal experience becomes “empirical data.” i.e. a burn on your hand is empirical data that the stove is hot.

Ground State: You’re not being lazy, you are in your ground state.
Extrapolation: A semi-educated guess is an extrapolation

Ideal Case: You aren’t ignoring details, you are taking the ideal case
Vanishingly small: A tiny amount is “vanishingly small” or “negligible.” Really small is “infinitesimal”

You aren’t overweight, you are thermodynamically efficient
Potential Well: Stuck in a meeting is “trapped in a potential well,” though you hope you can “tunnel out.”

Blackhole: If there is no escape, you are trapped by a black hole, from which there is no escape.

Photons: It’s not light, they are photons. Turning on the lamp becomes emitting photons.

Exercise to Reader: The rest is history becomes “the rest is left as an exercise to the reader…”

Not even wrong. Someone is making an argument using assumptions that are known to be wrong, or are making an argument that can’t be falsified. Courtesy Wolfgang Pauli. “Wait, he’s assuming Ron Paul can still win the Republican nomination? That’s not even wrong.”

For very small values of. This one, I’m afraid, I can best explain by example. “So there are four of us going to dinner.” “Three.” “Okay, so there are four of us for very small values of four.”

Super position: If something seems to act like something else, I say that it’s in a “superposition of the two states”.

Q: two trains are a approaching each other at 40 mph and are 8 miles apart, when will they cross each other? An untrained person would say “in about 6 minutes”.

A physicist would respond as:
If the trains are on the same track, they will not cross each other but run in to each other; when you say approaching each other, are the trains engines facing each other? they could go around the world and then cross each other, but for that I need to know the location of the trains. Also, I need to know the lengths of each of the trains to accurately calculate this, but in a trivially simple situation, the trains will cross each other in 6 minutes.
Again, make sure that the correct answer is always present at the end.

The U.S. House of Representatives on Wednesday approved a resolution introduced two days earlier that designates March 14, 2009 (3/14, get it?) as National Pi Day. It urges schools to take the opportunity to teach their students about Pi and “engage them about the study of mathematics.”

Zuck called them “lighthearted reminders about the importance of math and science education,” adding “this year we decided to put together an effort to see if we could use this as a mechanism to increase awareness for math and science education.”

All the arguments in favor of Pi Day also apply equally to Physics Day.

The text of the resolution is here;

Whereas the Greek letter (Pi) is the symbol for the ratio of the circumference of a circle to its diameter;

Whereas the ratio Pi is an irrational number, which will continue infinitely without repeating, and has been calculated to over one trillion digits;

Whereas Pi is a recurring constant that has been studied throughout history and is central in mathematics as well as science and engineering;

Whereas mathematics and science are a critical part of our children’s education, and children who perform better in math and science have higher graduation and college attendance rates;

Whereas aptitude in mathematics, science, and engineering is essential for a knowledge-based society;

Whereas, according to the 2007 Trends in International Mathematics and Science Study (TIMSS) survey done by the National Center for Education Statistics, American children in the 4th and 8th grade were outperformed by students in other countries including Taiwan, Singapore, Russia, England, South Korea, Latvia, and Japan;

Whereas since 1995 the United States has shown only minimal improvement in math and science test scores;

Whereas by the 8th grade, American males outperform females on the science portion of the TIMSS survey, especially in Biology, Physics, and Earth Science, and the lowest American scores in math and science are found in minority and impoverished school districts;

Whereas America needs to reinforce mathematics and science education for all students in order to better prepare our children for the future and in order to compete in a 21st Century economy;

Whereas the National Science Foundation has been driving innovation in math and science education at all levels from elementary through graduate education since its creation 59 years ago;

Whereas mathematics and science can be a fun and interesting part of a child’s education, and learning about Pi can be an engaging way to teach children about geometry and attract them to study science and mathematics; and

Whereas Pi can be approximated as 3.14, and thus March 14, 2009, is an appropriate day for `National Pi Day’: Now, therefore, be it

Resolved, That the House of Representatives–

(1) supports the designation of a `Pi Day’ and its celebration around the world;

(2) recognizes the continuing importance of National Science Foundation’s math and science education programs; and

(3) encourages schools and educators to observe the day with appropriate activities that teach students about Pi and engage them about the study of mathematics.

Order of Magnitude: Use terms like “orders of magnitude” to describe significant differences of scale.

Negligible: When something is small, say it is “negligible” non-zero, but negligible.

Infinitesimal: If it is really really small, say it is infinitesimal.

Non-trivial: For a physicist, nothing is ever hard or difficult – it is always “non-trivial”

First-order approximation : That is only a first-order approximation to a good cup of coffee… “The living room is clean. Well…at least to a first order approximation.”

Canonical: Use “canonical” when you mean “usual” or “standard.” As in, “the canonical example of talking like a physicist is to use the word ‘canonical.’”

Orthogonal: Use “orthogonal” to refer to things that are mutually-exclusive or can’t coincide. “We keep playing phone tag — I think our schedules must be orthogonal”

Empirical Data: Any actual personal experience becomes “empirical data.” i.e. a burn on your hand is empirical data that the stove is hot.

Ground State: You’re not being lazy, you are in your ground state.

Extrapolation: A semi-educated guess is an extrapolation

Ideal Case: You aren’t ignoring details, you are taking the ideal case

Vanishingly small: A tiny amount is “vanishingly small” or “negligible.” Really small is “infinitesimal”

Potential Well: Stuck in a meeting is “trapped in a potential well,” though you hope you can “tunnel out.”

Blackhole: If there is no escape, you are trapped by a black hole, from which there is no escape.

Photons: It’s not light, they are photons. Turning on the lamp becomes emitting photons.

Exercise to Reader: The rest is history becomes “the rest is left as an exercise to the reader…”

Not even wrong. Someone is making an argument using assumptions that are known to be wrong, or are making an argument that can’t be falsified. Courtesy Wolfgang Pauli. “Wait, he’s assuming Ron Paul can still win the Republican nomination? That’s not even wrong.”

For very small values of. This one, I’m afraid, I can best explain by example. “So there are four of us going to dinner.” “Three.” “Okay, so there are four of us for very small values of four.”

Super position: If something seems to act like something else, I say that it’s in a “superposition of the two states”.
Other good words to add to your vocabulary:

Other rules:When you are asked a question, think of improbable ways a statement could be true and then at the end make sure to give correct answer.
When a mere mortal is asked a question “Can a pig fly?” typically the answerer is “No.”
However a physicist will respond as follows:“They could if there was no gravity, or in interstellar space, or if they had wings, or if they were dropped off a cliff (though it would be a short flight). Generally the answer is NO.”Here is another example of the same rule:
Q: two trains are a approaching each other at 40 mph and are 8 miles apart, when will they cross each other? An untrained person would say “in about 6 minutes”.
A physicist would respond as:
If the trains are on the same track, they will not cross each other but run in to each other; when you say approaching each other, are the trains engines facing each other? they could go around the world and then cross each other, but for that I need to know the location of the trains. Also, I need to know the lengths of each of the trains to accurately calculate this, but in a trivially simple situation, the trains will cross each other in 6 minutes.
Again, make sure that the correct answer is always present at the end.

Thanks to Sean, JerseyBoy, Swans on tea, dr. Dev. Stephan at Live granades, James Cronen and many others who have commented on various blogs on this issue.